Working prototypes have demonstrated the feasibility and utility of solar powered aircraft. Many if not most solar powered aircraft, however, rely on the photovoltaic conversion of sunlight to electricity to power an electric motor-based propulsion system. Batteries (or electrolyzer, gas storage, and regenerative fuel cells) are carried onboard the aircraft to store electrical energy and keep the aircraft aloft during the night, so that flight time is not limited by fuel supply as it is on a conventional aircraft. It is widely recognized, however, that onboard voltaic batteries or electrolyzer, gas storage, and regenerative fuel cell systems impose a substantial weight burden for all aircraft, and especially for high-altitude, long range aircraft.
Another problem associated with photovoltaic power generation arrangements for aircraft, especially high-altitude, long-range aircraft, is the need to orient/point the photovoltaic solar cells to face the sun. Having wing-mounted arrays of solar panels can limit the efficiency of the collection of solar power, especially at dawn and dusk, as sunlight seldom strikes the solar panels “face on”. Thus in order to achieve a direct angle of impingement, the aircraft could be “banked” (i.e. laterally incline the aircraft, such as by elevating one wing or side higher or lower in relation to the opposite wing or side) in order to face the sun. This practice is disclosed by U.S. Pat. No. 4,415,133 to Phillips, as well as U.S. Pat. No. 5,518,205 to Wurst, et al. Conventional aircraft, however, cannot maintain straight flight at a large bank angle for extended lengths of time. Moreover, a related problem is the significant restriction on the latitude range over which aircraft may be flow, often seen with wing mounted solar energy collection means, i.e. photovoltaic solar cells, characteristic of the prior art. During winter, at higher northern latitudes, the maximum angle of the sun above the horizon may be relatively small, and thus the effective collection area of the wing surface may be severely restricted.
While the Phillips reference alternatively suggests that solar cells may be placed on a tilting panel within a transparent fuselage structure, this arrangement would require the inclusion of a cooling system for the inner located cells, with the associated weight and aerodynamic drag penalties. The cooling requirement discussed in Phillips for maintaining high efficiency of inside-mounted cells is a generic limitation common to all photo-voltaic solar cell powered aircraft. This same limitation precludes the practical use of solar cells at the focus of a high concentration factor solar collector, since excessive heating of solar cells leads to substantially reduced efficiency.
Furthermore, the efficiency of photovoltaic electric energy collection, storage, and utilization in the prior art is relatively limited. Photovoltaic arrays of high efficiency are very expensive and tend to lose efficiency at elevated temperatures, and thus are not practical to use at the focus of a high flux solar concentrator. The prior art system of photovoltaic electric energy collection, storage, and utilization has a relatively small power to mass ratio. Thus the aircraft must typically fly at an altitude high enough to be above the clouds, and to avoid winds with velocities much higher than the airspeed of the vehicle, as described in the Phillips reference. Because of its long endurance and limited weight-carrying ability, this type of vehicle is normally considered to be a pilotless aircraft.
Various ground based solar energy collectors and concentrators, and interfaces to heat storage media and heat engines are also known. A few examples include: U.S. Pat. No. 4,586,334 to Nilsson, and U.S. Pat. No. 6,487,859 to Mehos. The Nilsson patent discloses “ . . . a solar energy power generation system which includes means for collecting and concentrating solar energy; heat storage means; Stirling engine means for producing power”, and “ . . . the means for collecting and concentrating solar energy is a reflective dish; and the heat transfer means includes first and second heat pipes; the heat storage means is preferably a phase change medium . . . ” The Mehos patent discloses: “ . . . sodium heat pipe receivers for dish/Stirling systems”, and cites references demonstrating: “ . . . sodium vapor temperatures up to 790° C.” Additionally, U.S. Pat. No. 4,125,122 discloses a heat pipe receiving energy from a solar concentrator, U.S. Pat. No. 6,700,054B2 describes connecting to a Stirling engine, among other things, and U.S. Pat. No. 4,088,120 describes a parabolic trough with a heat pipe at the focus connected to a heat storage medium. None of these representative references, however, disclose how the solar energy generation and storage system can be made sufficiently lightweight that it would be able to provide for the overnight propulsion of a solar-powered aircraft.
In addition, the utility of LiH as a thermal energy storage medium, i.e. a “thermal battery,” is known, and is based on the very high thermal energy per unit mass characteristic of LiH. For example, the specific energy released in the cooling of one kg of LiH from 1200 K to 600 K is 1900 W-hr. In contrast, lithium ion electrical storage batteries contain less than 10% as much energy per kg. Even a Hydrogen-Oxygen recyclable fuel cell with associated electrolyzer and gas storage contains no more than approximately 1000 W-hr per kg. It is appreciated that no other known solid, liquid, (or gaseous, if the mass of the requisite container is accounted for) compound has as high a specific thermal energy content as LiH for this temperature range. One example of LiH used as a thermal energy storage medium is disclosed in U.S. Pat. No. 3,182,653 to Mavleos et al. and directed to a Lithium hydride body heating device that uses LiH as a phase change medium to store heat energy for use in providing warmth to a diver. The '653 patent, however, does not disclose how highly reactive LiH may be safely contained for long periods of time. Theoretically, pure LiH has an infinite hydrogen vapor pressure just beyond the melting point of LiH. Thus, a container of LiH constructed according to the Mavleos disclosure, for example, may explode upon reaching the melting point of LiH at about 700° C.
Accordingly, it is an object of the present invention to provide an aircraft powered by the heat of the sun.
Another object of the present invention is to provide a lightweight and highly efficient solar power plant and system for powering an aircraft by the heat of the sun.
Another object of the present invention is to provide an internally mounted solar power plant and system for powering an aircraft which does not require internal cooling.
And another object of the present invention is to provide a means for efficiently powering a solar aircraft by using a high efficiency heat engine, such as a Stirling engine.
Another object of the present invention is to provide a means for storing sufficient solar energy accumulated during the day to enable flight through the nighttime without excessive mass burden.
Another object of the present invention is to provide a means for maximizing solar energy collection and concentration by optimally aligning a heat collection element to the sun without re-orienting or otherwise changing the flight characteristics of the aircraft, e.g. banking.
Another object of the present invention is to provide a means for conserving heat energy during night time operation by preventing backflow of a heat transfer working fluid of a heat pipe.
These objects are achieved by the present invention described hereinafter.